Systems, such as OpenAjax HUB, in which messages in a Publish/Subscribe communication model are communicated in an asynchronous manner, have been developed (for example, see Japanese Laid-open Patent Publication No. 2007-213372). Such a system allows decoupling the request side of event notification (Subscribe side: event receiving side) and the event notification side (Publish side: event publishing side), which is the side opposite to that request side, so that their respective operations are defined in applications independently of each other.
Recently, there has been an increasing shift to portals using gadgets (widgets) developed in compliance with common specifications typified by OpenSocial. Such a portal allows the user to freely combine gadgets to create content. For communication between gadgets, for example, event notification mechanisms in the Publish/Subscribe model such as OpenAjax HUB are used.
However, in a model having two phases, publishing an event and receiving the event, such as in the Publish/Subscribe model, the time at which event notification is requested (Subscribe) and the time at which, in response to this request, event notification is published (Publish) are not necessarily close in terms of time. Therefore, in some cases, the order of priority of event notification differs between the stage in which event notification is requested and the stage in which the requesting source is actually notified of the event. Consequently, if, according to the order of priority at the time at which event notification is requested, the requesting source is notified of an event after the event is published, notification of the event is sometimes performed in an order inappropriate for the time at which event notification is published although the order is appropriate for the time at which event notification is requested. In view of the above, it is desirable that event notification be performed in an appropriate order.